In two stroke engines with one or several high efficiency cylinders, there is currently an independent attempt to embody a scavenging of a cylinder or cylinders by non-carburated fresh air and a means to introduce atomized liquid fuel into the cylinder(s), both these operations being effected at successive times and properly determined in accordance with the operating cycle of the engine.
The introduction of atomized fuel into the cylinder may be effected by a pneumatic injection device comprising an injector opening into the cylinder provided with a valve adapted to be opened and closed by a cam, a device for feeding the injector with liquid fuel and a compressed air source ensuring atomization and the injection of fuel when the injector is opened.
The fresh air scavenging of the cylinder may be effected with the aid of a pump housing communicating with the cylinder at its lower portion so that the piston moving inside the cylinder compresses the air of the housing by moving towards its bottom dead center position. Pipes joining the housing to scavenging ports of the cylinder ensure transfer of the compressed air to the cylinder, with the compressed air penetrating into the cylinder its scavenges when the scavenging ports are uncovered by the piston as the piston moves towards a bottom dead center position.
The pneumatic injection of fuel is effected by using the compressed air in a pump housing to atomize and inject the fuel. To this end, the pump housing may be connected to the injector by a pipe having a valve. The portion of the pipe situated downstream of the valve may form a capacitor. When the injector is opened, a certain quantity of compressed air is used to atomize the fuel and inject the atomized fuel into the cylinder. The recharging of the capacitor with compressed air is effected by opening the valve when the pressure approaches it maximum in the pump housing.
FR-A-2,625,532 and corresponding U.S. Pat. No. 5,027,765 describes an injection method in which the atomization and injection of fuel into a cylinder of a two stroke engine are effected by using gases taken from a different engine cylinder in which injection is effected.
In one particular embodiment, the pneumatic fuel injector may be fed with gas under pressure by a storage capacitor connected to the chamber of the cylinder in which injection takes place by the chamber of the pneumatic injector opening into the upper portion of the cylinder at the receptacle of a closing and opening valve.
Thus, increased performances are obtained. However, two stroke engines operating according to known injection methods of the prior art does not always make it possible to obtain sufficient performances, especially when they operate on high charges.
At the time of injection, the air/fuel ratio is often inadequate so as to obtain good fuel atomization and effective combustion. Generally speaking, it is not possible to supercharge the engine in order to increase the torque.
The conditions for implementing distribution also result in limitations as regards the operating speed of the engine.
The valves for admitting the mixture need to be relatively large so as to be able to use a cylinder head as high as the valves.